An optical transmission link is typically composed of an optical transmit-and-receive device, wherein the function of a converter or a repeater amplifier can also be integrated into this transmit-and-receive device, at each end point of the optical link that can be composed of one or more optical waveguides. In this way, incoming signals, which could be of an optical or electrical type, are amplified, processed, or converted, in order to further process or forward them. Such an optical transmit-and-receive device can be connected, for example, between a local-area network (LAN) and a wide-area network (WAN) in order to shift the data transmission from one optical wavelength to another. Likewise, such a transmit-and-receive device can also be used for signal processing or as an amplifier unit within wide-area networks. Likewise, optical transmit-and-receive devices of this type can also be used to convert electrical signals fed from the outside into optical signals and vice versa. Usually, two or more such links (sections of optical transmission links with two transmit-and-receive devices at the end points of an optical link) are coupled to form an entire transmission link.
The optical transmission is typically performed via optical waveguides at wavelengths of, for example, 1310 nm in local-area networks or 1550 nm for greater distances. In this way, transmission rates of currently up to several 10 Gbps are achieved. Transmitted and received data can be transmitted within a link on a common optical waveguide or on separate optical waveguides. For shared transmission on one optical waveguide, signals to be transmitted and received are separated with a selective coupler before the input and after the output, respectively, of a transmit-and-receive station or also within such a station.
Various methods for checking or for starting up transmission links, in particular, links of transmission paths, are known. They are carried out during the setup of data-transmission devices or also after data transmission has been interrupted, in order to ensure that proper communications can be established between two or more transmit and receive stations.
For the optical data transmission, high transmit powers are used, especially in wavelength division multiplex systems (WDM systems). The light signals that are emitted in this way and that are transmitted from one station to the other can be dangerous to the human eye if the eye is exposed to such radiation for a certain duration. This can happen, for example, when an optical waveguide that is in use is severed during roadwork or underground work and a third party examines the damaged cable. Likewise, through intentional breaking of the connection, such as, for example, when detaching a plug connection of an optical waveguide, this radiation can be emitted and can enter the human eye. To counteract the risk of injury to the eye, according to known methods, it is typical to stop the transmission operation immediately after the detection of an interruption if a line breaks or if there is some other unintentional interruption of the connection.
For reestablishing the transmission operation after such an interruption or for the first-time setup, initially the functionality of the transmission link must be tested.
In German Patent Application No. DE 44 36 131 A1 a method is presented that uses a so-called dummy telegram for such function testing. Disadvantageously, the method must work with a minimum signal period, and the time-intensive transmission of several dummy telegrams one after the other in the same direction is also required.
In practice, function testing usually takes place in such a way that test signals (for example, pulse trains) are transmitted and the type and duration of these signals (for example, pulse duration shorter than 5 ms) are set by laser protection classes. In the sense of a query, a test signal is introduced into the transmission link by a first transmit-and-receive station, in order to be received by a second such station when the transmission path is intact, at least in this direction.
When such a test signal is received and evaluated, the second station transmits this same signal as an answer signal back in the other direction of the transmission link to the first station. The first station assumes the transmission mode only after receiving an incoming signal interpreted as a corresponding answer, because the transmitted test signal and a subsequently arriving answer signal are considered evidence that a transmission link is operating according to regulations.
In the state of the art, such test signals are transmitted at given time intervals and with a fixed time duration of, for example, 2 ms. Within a certain time span (time window) after transmitting a test signal, a signal must appear as an answer from the opposing station, in order to indicate the functionality of the transmission link. The test and answer signals do not differ from each other; all that is important is that the querying station receive an answer signal within the given time span after transmitting the test signal.
However, here the disadvantage is the risk that a signal of a first station understood as an answer was actually only a test signal that was transmitted by a second station, in order to test, on its side, the functionality of the transmission link. In this case, the first station would assume the transmission mode after the apparent confirmation of the functionality by the opposing station, even though it was possible that the test signal transmitted by it never reached the second station—due, for example, to a break in the line of an optical waveguide. Then high-energy radiation that could possibly be dangerous would be emitted at the location of the break.
In practice, this problem should be minimized in that the time points at which a station transmits a test signal and the time span within which an answer signal is expected are set randomly. This should prevent the situation that individual stations transmit test signals that could be mistakenly understood as answer signals at possibly the same cycles and, in the most unfavorable situation, at similar time points. However, this solution does not offer total security from such “misunderstandings,” because even transmit time points selected at random could lie so close to each other that incoming signals could be interpreted as answer signals.
Here, the method must also disadvantageously work with a very narrow time window after transmission of a test signal within which a detection of a signal as an answer signal is possible. This increases the technical expense and the error susceptibility of the system. In addition, the maximum length of the transmission link is limited by this time window and the signal propagation time or the signal processing time.
For test purposes, such a transmit-and-receive station can also be switched (“looped”) into loop mode. Here, for example, the optical input is directly connected to the optical output, so that a received signal is transmitted back in the same direction without evaluation or processing. Such a loop can also be connected so that incoming optical signals are first converted into electrical signals and then back into optical signals before they are transmitted back in the same direction. Finally, in a loop circuit, electrical processing of incoming signals with respect to timing recovery and bit pattern is also possible before the signals are transmitted back. In the loop mode, however, the incoming signal is typically not tested or evaluated. A loop circuit of a second transmit-and-receive device that is connected to a first transmit-and-receive device can be advantageous, for example, for measuring properties of the transmission link, such as propagation time or signal-to-interference ratio.
The test method for testing the functionality of a transmission link according to the state of the art also functions when a station is switched into loop mode, because the incoming test signal is then immediately transmitted back as the answer signal.
In so-called Open Fibre Control according to the ANSI standard, both transmit-and-receive devices at the end points of a link transmit a pulse of 617 μs long every 10.1 sec independently of each other. If the first transmit-and-receive device receives a pulse from the second transmit-and-receive device, the second transmit-and-receive device immediately transmits a pulse 617 μs long. This pulse transmitted by the second transmit-and-receive device must arrive at the first transmit-and-receive device before the query pulse transmitted by this first device has ended. This method is executed twice, wherein, in this second execution, the transmit-and-receive devices continue to transmit as long as a receive signal is detected from the other transmit-and-receive device. In this way the connection is established.
The essential disadvantage of this method is that, due to the pulse length of 617 μs, the roundtrip time and thus the length of the link is limited to approximately 60 km. If a delay time for detecting the pulses is also considered, this would produce, in practice, a limit to the maximum link length of approximately 35 km.
German Patent Application DE 100 58 776 C1 describes a method for testing the functionality of an optical transmission link, wherein this method works with different lengths of query and answer pulses. This avoids limiting the link length. A disadvantage, however, is the evaluation of the pulse length. This evaluation produces corresponding expense. In addition, the pulse length can be shortened by optical amplifiers and regenerators in the link, so that incorrect interpretations are possible.